low altimeter and FL180

[Michael 182]

Was flying in the Denver area today and approach told a plane that FL 180
was unavailable die to low altimeter, which was 28.87 at the time. What is
the reason for this? Confusion with 29.92 readings?

Thanks,

Michael

[Steven P. McNicoll]

"Michael 182" > wrote in message
news:V3Bnb.36695$mZ5.183695@attbi_s54...
>
> Was flying in the Denver area today and approach told a plane that FL 180
> was unavailable die to low altimeter, which was 28.87 at the time. What is
> the reason for this? Confusion with 29.92 readings?
>

Because there would be less than 1000 feet separation between aircraft at
17,000 MSL using 28.87 as an altimeter setting and aircraft at FL180 using
29.92. In this case, FL190 is also unavailable for the same reason.

[Frank Ch. Eigler]

"Michael 182" > writes:

> Was flying in the Denver area today and approach told a plane that FL 180
> was unavailable die to low altimeter, which was 28.87 at the time. What is
> the reason for this? Confusion with 29.92 readings?

FL180 is 18000 indicated feet with the altimeter set to 29.92. That
same airspace region is at 16950 ft indicated with the actual
altimeter setting. In order to keep traffic using the two altimeter
settings apart, FL180 is not used, nor is FL190 (17950) in this case.

- FChE

[Ron Natalie]

"Frank Ch. Eigler" > wrote in message ...
>
> "Michael 182" > writes:
>
> > Was flying in the Denver area today and approach told a plane that FL 180
> > was unavailable die to low altimeter, which was 28.87 at the time. What is
> > the reason for this? Confusion with 29.92 readings?
>
> FL180 is 18000 indicated feet with the altimeter set to 29.92. That
> same airspace region is at 16950 ft indicated with the actual
> altimeter setting. In order to keep traffic using the two altimeter
> settings apart, FL180 is not used, nor is FL190 (17950) in this case.
>
To put it another way, flight levels don't exist below the transition altitude
(18,000 Feet MSL In the US). If FL180 would be at 16950, it's not available.

[Michael 182]

Thanks for the replies. Makes sense now.

Michael

"Ron Natalie" > wrote in message
m...
>
> "Frank Ch. Eigler" > wrote in message
...
> >
> > "Michael 182" > writes:
> >
> > > Was flying in the Denver area today and approach told a plane that FL
180
> > > was unavailable die to low altimeter, which was 28.87 at the time.
What is
> > > the reason for this? Confusion with 29.92 readings?
> >
> > FL180 is 18000 indicated feet with the altimeter set to 29.92. That
> > same airspace region is at 16950 ft indicated with the actual
> > altimeter setting. In order to keep traffic using the two altimeter
> > settings apart, FL180 is not used, nor is FL190 (17950) in this case.
> >
> To put it another way, flight levels don't exist below the transition
altitude
> (18,000 Feet MSL In the US). If FL180 would be at 16950, it's not
available.
>
>

[BTIZ]

AIM Section 7 Chapter 2

"Michael 182" > wrote in message
news:V3Bnb.36695$mZ5.183695@attbi_s54...
> Was flying in the Denver area today and approach told a plane that FL 180
> was unavailable die to low altimeter, which was 28.87 at the time. What is
> the reason for this? Confusion with 29.92 readings?
>
> Thanks,
>
> Michael
>
>

[Robert Lyons]

Ron Natalie wrote:

> "Frank Ch. Eigler" > wrote in message ...
>
>>"Michael 182" > writes:
>>
>>
>>>Was flying in the Denver area today and approach told a plane that FL 180
>>>was unavailable die to low altimeter, which was 28.87 at the time. What is
>>>the reason for this? Confusion with 29.92 readings?
>>
>>FL180 is 18000 indicated feet with the altimeter set to 29.92. That
>>same airspace region is at 16950 ft indicated with the actual
>>altimeter setting. In order to keep traffic using the two altimeter
>>settings apart, FL180 is not used, nor is FL190 (17950) in this case.
>>
>
> To put it another way, flight levels don't exist below the transition altitude
> (18,000 Feet MSL In the US). If FL180 would be at 16950, it's not available.
>
>



I hope you'll forgive the intrusion from a lurker and non-pilot (well,
a wannabe-pilot). Would someone please explain a few points about this
to me? I understand that altimeters are re-calibrated to correct for
the current barometric pressure (extrapolated to MSL). My questions:

- Why would people be using two different altimeter settings anyway?
- Why would this problem affect just FL180 and FL200? Why not FL160, 220,
and every other altitude?
- Portions of other posts make it sound like there's something special
about calibrating ones' altimeter to 29.92, as if that's commonly done
in certain situations. Is there something different about that specific
number? or am I misinterpreting other posts?

I don't have any reference books mentioned in another post, so cannot
look up these answers myself. Thanks for any light you can shed.

- Bo

[G.R. Patterson III]

Robert Lyons wrote:
>
> - Why would people be using two different altimeter settings anyway?

Because the regulations require it. In the States, all aircraft flying at or
above 18,000' ASL are required to set their altimeters to 29.92. Aircraft flying
below 18,000' are expected to set their altimeters to the local barometric
pressure.

> - Why would this problem affect just FL180 and FL200? Why not FL160, 220,
> and every other altitude?

It only affects flight levels at which the actual altitude of a plane using the
29.92 setting would be low enough to conflict with aircraft using the actual
pressure setting. In this case, a plane flying at 18,000' using 29.92 would be
at 16950 ft indicated with the actual altimeter setting. If another aircraft
using the local barometric pressure is flying at 17,000', there is a possibility
of collision. One pilot "knows" he's at 18,000' and the other pilot "knows" he's
at 17,000', but they're actually at nearly the same altitude. Since altimeters
can be slightly inaccurate, they could easily be at exactly the same altitude.

Since ATC is tasked with ensuring a certain amount of separation between IFR
aircraft, ATC will simply not assign 18,000' as a flight altitude.

FL 200 and higher don't present problems here. FL 210 would be an altitude of
19,950', and there will be no aircraft at that altitude using the local setting.

George Patterson
You can dress a hog in a tuxedo, but he still wants to roll in the mud.

[Peter Duniho]

"Robert Lyons" > wrote in message
...
> - Why would people be using two different altimeter settings anyway?

Well, the situation at hand is that one airplane is flying at or above
"flight level 180" while the other is flying below "flight level 180". By
definition, altitudes "in the flight levels" are referenced to standard
atmospheric pressure (29.92) even if non-standard conditions prevail. So
anyone flying at or above FL180 has their altimeter set to 29.92. However,
for all flying below the flight levels, the altimeter will generally be set
to the prevailing conditions (sea-level corrected, of course).

This is, in fact, the whole point with respect to the question at hand.

The reason for using a single altimeter setting for all airplanes at or
above FL180 is that most aircraft flying at that altitude travel so quickly
that it would be impractical for them to have to keep changing their
altimeter setting as they fly through different weather systems. They are
at a high enough altitude that the error between indicated and true altitude
won't cause any problem with terrain (at least in the US), which is the main
reason for using a local altimeter setting in the first place.

> - Why would this problem affect just FL180 and FL200? Why not FL160, 220,
> and every other altitude?

It affects all flight above FL180. There is no "FL160" in the US, and I'm
not sure why you got the impression that flights at FL220 are unaffected.

> - Portions of other posts make it sound like there's something special
> about calibrating ones' altimeter to 29.92, as if that's commonly done
> in certain situations. Is there something different about that specific
> number? or am I misinterpreting other posts?

29.92" is the pressure at sea level in standard atmospheric conditions. It
just happens to be a nice "middle-ground" number. Nothing special other
than that.

By the way, we typically reserve the word "calibrate" for an infrequent
operation in which the accuracy of the altimeter is ensured. Setting the
altimeter setting to a specific number in flight is generally referred to as
"setting the altimeter setting". :)

Pete

[Robert Lyons]

Thanks VERY much, George and Peter, for your cogent answers to my
questions. I understand now.

- Bo

[Montblack]

No intrusion, just a great question. Keep reading ...and posting.

Now you have the info others have (links). These resources are free online,
or you can buy a printed FAR/AIM 2004 book for under $20.

http://www1.faa.gov/ATPubs/AIM/index.htm
Aeronautical Information Manual
Official Guide to Basic Flight Information and ATC Procedures

http://www.landings.com/evird.acgi$pass*58241480!_h-www.landings.com/_landings/pages/regulations.html#aim
Federal Aviation Regulations

or

http://makeashorterlink.com/?L2EF42F56
(Same as long link above. Wait a few seconds for it)

These are from Landings.com. You can also get them from the FAA website.

--
Montblack
"Just the usual inanity"


("Robert Lyons" wrote)
> I hope you'll forgive the intrusion from a lurker and non-pilot (well,
> a wannabe-pilot). Would someone please explain a few points about this
> to me?

> I don't have any reference books mentioned in another post, so cannot
> look up these answers myself. Thanks for any light you can shed.

[David Brooks]

"Peter Duniho" > wrote in message
...
> "Robert Lyons" > wrote in message
> ...

> > - Why would this problem affect just FL180 and FL200? Why not FL160,
220,
> > and every other altitude?
>
> It affects all flight above FL180. There is no "FL160" in the US, and I'm
> not sure why you got the impression that flights at FL220 are unaffected.

It'd have to be a hell of a depression to affect FL220 though. The table in
the relevant regulation (14CFR91.121) doesn't allow for such sucky weather.

-- David Brooks

[David Brooks]

"G.R. Patterson III" > wrote in message
...
>
>
> Robert Lyons wrote:
> >
> > - Why would people be using two different altimeter settings anyway?
>
> Because the regulations require it. In the States, all aircraft flying at
or
> above 18,000' ASL are required to set their altimeters to 29.92. Aircraft
flying
> below 18,000' are expected to set their altimeters to the local barometric
> pressure.

I have occasionally idly wondered about one little detail. When it's a
significantly low or high pressure day, when do you change the altimeter
setting while climbing or descending? Maybe as you pass through FL180
descending, and as you pass through 18,000ft climbing?

Or maybe you do it when you get the clearance through the transition
altitude, to make it easier to plan the climb/descent (assuming the computer
isn't doing all that for you). Or maybe it takes you so long to twist the
knob that it doesn't much matter. What do you oxygen-suckers actually do?

-- David Brooks

[David Brooks]

"Ron Natalie" > wrote in message
m...

> To put it another way, flight levels don't exist below the transition
altitude
> (18,000 Feet MSL In the US). If FL180 would be at 16950, it's not
available.

Is there a similar regulation in countries with lower transition altitudes?
On a really high pressure day, could you be cleared from 2,000ft to FL000?

-- David Brooks

[Peter Duniho]

"David Brooks" > wrote in message
...
> I have occasionally idly wondered about one little detail. When it's a
> significantly low or high pressure day, when do you change the altimeter
> setting while climbing or descending? Maybe as you pass through FL180
> descending, and as you pass through 18,000ft climbing?

Most of the planes flying in the flight levels, they climb quickly enough
that asking "when do you change the altimeter" is like asking "should I have
my dinner delivered at 5 minutes, 31 seconds past the hour, or 5 minutes 37
seconds past the hour". :)

For the rest of "us" (my plane is certified to 20,000', though it's not
worth the trouble to fly that high), I believe that you're supposed to
switch over to 29.92 during the climb when you actually reach FL180, and
switch back to the local altimeter during the descent when you leave FL180.

When you actually do switch is, of course, more a matter of practicality
than anything, since you won't be given vertical separation during the climb
or descent that might depend on your altimeter setting (not counting
separation at the top or bottom of the climb or descent, of course).

Pete

[Peter Duniho]

"David Brooks" > wrote in message
...
> It'd have to be a hell of a depression to affect FL220 though. The table
in
> the relevant regulation (14CFR91.121) doesn't allow for such sucky
weather.

I guess that depends on your definition of "affect". The local barometric
pressure *always* affects FL220, just as it affects all other altitudes. If
you mean that FL220 is unlikely to be below 18,000' true altitude, sure.
But it moves around just as much as all the other flight level altitudes
(altitudes not really being discrete intervals).

Pete

[Robert Moore]

"David Brooks" wrote
> Maybe as you pass through FL180 descending, and as you pass through
> 18,000ft climbing?

You got it!

Bob Moore

[BTIZ]

The US uses 18,000 or FL180 as it's "transition" altitude.. other countries
us other altitudes, some as low as FL040 or 4,000ft MSL.

In the US: reset to 29.92 when climbing upon passing 18,000ft and climbing
to a FL
reset to the assigned local setting as soon as starting a descent when the
"altitude cleared to" is below FL180.

BT

"David Brooks" > wrote in message
...
> "G.R. Patterson III" > wrote in message
> ...
> >
> >
> > Robert Lyons wrote:
> > >
> > > - Why would people be using two different altimeter settings anyway?
> >
> > Because the regulations require it. In the States, all aircraft flying
at
> or
> > above 18,000' ASL are required to set their altimeters to 29.92.
Aircraft
> flying
> > below 18,000' are expected to set their altimeters to the local
barometric
> > pressure.
>
> I have occasionally idly wondered about one little detail. When it's a
> significantly low or high pressure day, when do you change the altimeter
> setting while climbing or descending? Maybe as you pass through FL180
> descending, and as you pass through 18,000ft climbing?
>
> Or maybe you do it when you get the clearance through the transition
> altitude, to make it easier to plan the climb/descent (assuming the
computer
> isn't doing all that for you). Or maybe it takes you so long to twist the
> knob that it doesn't much matter. What do you oxygen-suckers actually do?
>
> -- David Brooks
>
>

[Paul Sengupta]

3000ft here in the UK. Unless specified otherwise:

http://www.caa.co.uk/docs/64/DAP_ACD_8_Altimeter.pdf

Paul

"BTIZ" > wrote in message
news:zqYnb.103692$La.22441@fed1read02...
> The US uses 18,000 or FL180 as it's "transition" altitude.. other
countries
> us other altitudes, some as low as FL040 or 4,000ft MSL.

[GLPILOTSRV]

In article >, "David Brooks"
> writes:

>What do you oxygen-suckers actually do?

Reset the altimeter passing through FL180 going up or down. However, I hope I
am not sucking oxygen as this would be indicative of a major malfunction. :-)

G. Lee

[Robert Moore]

"Paul Sengupta" wrote

> In the UK you use altitude when descending, FL when climbing.

Paul, I have a hard time believing that. As one descends with
the altimeter set to 29.92, how would you know when you reach
or pass an "altitude"? Altitudes are determined by the local
altimeter setting, not 29.92.

Bob Moore

[Paul Sengupta]

Often wondered that myself. I change over when I see the
houses or trees at a certain size... :-)

Paul

"Robert Moore" > wrote in message
. 7...
> how would you know when you reach
> or pass an "altitude"? Altitudes are determined by the local
> altimeter setting, not 29.92.

[GLPILOTSRV]

In article >,
(GLPILOTSRV) writes:

>Reset the altimeter passing through FL180 going up or down. However, I hope I
>am not sucking oxygen as this would be indicative of a major malfunction. :-)
>

Ah, I forgot to mention that I do this in the continental US. The rules, of
course, are different alsewhere.

G. Lee

[Robert Moore]

(GLPILOTSRV) wrote
>
>>Reset the altimeter passing through FL180 going up or down.
>>However, I hope I am not sucking oxygen as this would be
>>indicative of a major malfunction. :-)
>>
>
> Ah, I forgot to mention that I do this in the continental US.
> The rules, of course, are different alsewhere.

How do you know that you are passing FL180 on the way up?

Bob Moore

[GLPILOTSRV]

In article >, Robert Moore
> writes:

>Reset the altimeter passing through FL180 going up or down.
>>>However, I hope I am not sucking oxygen as this would be
>>>indicative of a major malfunction. :-)
>>>
>>
>> Ah, I forgot to mention that I do this in the continental US.
>> The rules, of course, are different alsewhere.
>
>How do you know that you are passing FL180 on the way up?
>

Ok, let's try this. In the continental US, I set the altimeter to 29.92 at or
near 18,000' in the climb and passing through about FL180 in the descent. No
conflict with traffic should occur since an aircraft will not be assigned and
altitude of FL180 if the current barometric pressure is less than 29.92. It is
not necessary to be exacting in this. Just be sure to have the correct
altimeter setting in the window when approaching the assigned altitude to avoid
traffic and/or terrain conflicts.

However, should I desire to know that I am passing FL180 in the climb, and
since I a have three altimeters at my disposal, I could set one to 29.92 to
indicate my current FL.

G. Lee

[Robert Moore]

(GLPILOTSRV) wrote

> However, should I desire to know that I am passing FL180 in the
> climb, and since I a have three altimeters at my disposal, I
> could set one to 29.92 to indicate my current FL.

If you are not assigned FL180 why would you want to know when you
are passing it in a climb?

Bob Moore

[Garner Miller]

In article >, Robert
Moore > wrote:

> If you are not assigned FL180 why would you want to know when you
> are passing it in a climb?

So you know when you reset your altimeter so you don't go through the
190 you were assigned. A few years back we had a day with an absurdly
low local altimeter setting (something like 29.02).

We were climbing to FL190, so as we're passing through just under
18,000 feet and climbing like a bat outta hell, I whirl the altimeter
up to 29.92 to find we (ahem...) almost climbed through our altitude.
There were only 200 feet between 17,900 feet and FL190. Quite an eye
opener, to say the least.

--
Garner R. Miller
ATP/CFII/MEI
Manchester, CT =USA=